Massive flames are seen devouring both vehicles just after a crash Thursday when a FedEx tractor-trailer crossed a grassy freeway median in Northern California and slammed into a bus carrying high school students on a visit to a college.(Photo: Jeremy Lockett, AP)

U.S. safety investigators say the driver of a FedEx tractor-trailer that struck a bus carrying high school students didn't appear to brake before a fiery collision that left 10 dead, and that there was no evidence that the truck was on fire before the crash.

National Transportation Safety Board member Mark Rosekind said Saturday that the truck left no tire marks as it careened across a median and slammed head-on into the bus taking the students to a college tour in Northern California. On Sunday, the NTSB said it had not found evidence that the truck was burning before the crash, contrary to what witnesses Bonnie and Joe Duran told two TV stations.

The couple were in a Nissan Altima in front of the bus and told KNBC-TV Los Angeles they saw flames behind the truck cab before it crossed the median into the path of the tour bus Thursday evening on Interstate 5 near Orland, Calif.

"I just looked to the left, and there it was coming through right at me at an angle. I can tell I wasn't going to outrun him, so I just kind of turned to the right and he hit me," she said. "It was in flames as it came through the median. … It wasn't like the whole thing was engulfed. It was coming up wrapping around him."

National Transportation Safety Board member Mark Rosekind said investigators are not ruling out a pre-impact fire in the FedEx truck, but hadn't found any evidence at the crash site.

The truck hit the back door and trunk of the car before crashing into the charter bus filled with a dozen high school students and their chaperons.

Initial reports by police indicated the truck swerved to avoid a sedan that was traveling in the same direction in this town about 100 miles north of Sacramento, then went across the median. Officer Lacey Heitman, a spokeswoman for California Highway Patrol, said she could not confirm if the truck was on fire before the collision until all evidence was gathered.

Federal investigators also recovered an electronic control module from the bus, but do not know what information it contains. They were not able to recover such equipment from the FedEx truck tractor, a 2007 Volvo, but may be able to calculate speed and maneuvering using the transmission and marks in steering box.

But in addition to the cause of the crash, federal transportation authorities are examining whether fire safety measures they previously recommended for motor coaches could have allowed more of the 48 bus occupants to escape unharmed.

As part of its investigation, the NTSB said it is trying to determine whether the FedEx driver might have fallen asleep or suffered a health problem and whether there were mechanical issues with the truck. The agency also is evaluating whether there should have been a barrier on the median to help prevent head-on collisions. Barriers are required when medians are less than 50 feet wide; this one was 60.

Silverado Stages, the San Luis Obispo, Calif.-based company that owns the charter bus involved in the crash, has a strong safety record, and it has said it is fully cooperating with the investigation. It is unclear what sort of fire-safety equipment the bus in Thursday's crash had, and the company couldn't be reached for comment Saturday.

The crash killed five teens, three adult chaperons and the drivers of both the truck and the bus. The CHP confirmed more than 30 people were injured, Humboldt State said in a statement.

Among the dead were newly engaged couple Michael Myvett and Mattison Haywood, who had agreed to chaperon the 44 teenagers from Southern California on a visit to Humboldt State University. Myvett, who graduated from Humboldt State with a psychology degree in 2007 and worked with autistic children, was eager to make the trip with his fiancée.

Haywood was "the love of his life," and "to be a liaison and representative for high school students who wanted to attend Humboldt was in sync with his personality, wanting to facilitate peoples' achievement of their dreams," Kyle Farris, a colleague at the Center for Autism and Related Disorders in Torrance, told the Associated Press.

Among the high school students were 17-year-old identical twins Marisol and Marisa Serrato, who traveled on separate buses. Marisol's bus arrived safely. Marisa died in the crash, her brother Miguel Serrato told AP.

"Marisol is devastated," he said.

Adrian Castro, 19, a senior and a football player at El Monte High School east of Los Angeles, had considered going to a state university closer to home but decided to give faraway Humboldt, home of sports teams known as the Lumberjacks, a chance, his father, Raul Castro, told the San Gabriel Valley Tribune.

Raul Castro dropped his son off to meet the bus on Thursday morning. Later that night, he got a call from Adrian's mother, who said she had heard from the California Highway Patrol that the young man had died.

Humboldt admissions counselor Arthur Arzola, 26, who recruited for the school in the Los Angeles area, died of burns sustained in the crash. On Humboldt's webpage, Arzola described himself as "hardworking, thoughtful, compassionate and friendly."

Colleagues remembered Arzola for "his passionate commitment to helping low-income and first-generation students get into college," the university said. "He dedicated his career to that work."

The teenagers, who hailed from dozens of Southern California high schools, were traveling as part of the university's Preview Plus program, which began in 1989 as a way to introduce low-income or first-generation prospective college students to the university. The students would live in dorms for the weekend, explore the campus with current college students and learn about the university's offerings.

"Our hearts go out to the families, loved ones, classmates and communities of everyone touched by this tragedy," Education Secretary Arne Duncan said Friday. "I can only imagine the excitement of these high school students as they were on their way to visit a college campus, and the pride of the adults who were accompanying them. Our young people are our greatest treasure, and this loss is heartbreaking."